1. Field of the Invention
The present invention relates to a PPI radar apparatus for use in shipping, aircraft, etc. and, more particularly, to a PPI radar appartus adapted to display an image of a target by the switchover of distance ranges.
2. Description of the Prior Art
In the conventional display of a received video signal in a radar apparatus in accordance with a polar coordinate sweep system (hereinafter referred to as a "PPI" system), transmission of a pulse by a radar antenna and the start of a sweep signal are carried out at the same time and a video signal, received with a delay corresponding to the radiowave propagation distance is, fed to a CRT display under real-time operation to thereby display a target image.
For this reason, with respect to the switchover of the distance ranges in the display on a CRT screen, it is necessary to sweep time and sweep repetition frequency in accordance with the length of the distance.
When the sweep time is varied by the switchover of the distance range, however, the display luminance brightness of the CRT screen varies. More particularly in the short-distance range in which the sweep time is shortened, the display screen becomes dark. On the other hand, the distance range is generally divided into nine or ten ranges to be swtiched over. These ranges each require separate circuits for generating different-time sweep waveforms and, therefore, there gives rise to complexity of a circuit system and an increase in cost. Since the power density for the deflection drive of the CRT in a short-time sweep within a short distance range is increased, the service life of the CRT display is shortened. Further, since the short-time sweep waveform is inferior in linearity, an expensive circuit is required in order to secure linearity.